1. Field of the Invention
The present invention relates to wireless data communication technology; and particularly to a system and method for contention control in wireless network.
2. Description of the Related Art
To support wireless local area network (WLAN) applications with Quality of Service (QoS) requirements, the IEEE 802.11e supplement has been introduced. The 802.11e includes an additional channel access method called hybrid coordination function (HCF), which is usable in Quality of Service (QoS) network configuration. HCF is upwardly compatible with distributed coordination function (DCF) and point coordination function (PCF), and at the same time provides QOS-enabled stations (QSTAs), in which Quality of Service is supported, with prioritized and parameterized QoS access to the wireless medium. HCF combines aspects of both contention-based (EDCF) and polling-based (PCF) channel access mechanism.
In PCF mode, a hybrid coordinator (HC) dominates channel access determination. The HC may be an access point (AP) or a station (if an AP is absent). The winning station in EDCF mode or the polled station in PCF mode obtains a transmission opportunity (TXOP) to transmit the packets. The HC gains control of the channel after sensing the idle channel for a time period equivalent to Point coordination function Inter-Frame Spaces (PIFS). PIFS is used by the HC to take control of the channel and start a PCF. After grabbing the channel, the HC polls a QSTA according to its polling list. The polled station may carry out several packet exchange sequences during one TXOP. At the end of a TXOP, the HC gains control of the channel again, and it either sends a QoS-Poll to the next QSTA on its polling, or releases the channel if there are no more stations to be polled.
The HC controlled contention (CC) mechanism determines which transmission time slot is occupied for a particular QSTA and allows QSTAs to request TXOPs without having to contend with EDCF traffic. These requests may be used to initiate either periodic polled TXOPs for traffic of periodic type, or one-time TXOPs to handle burst traffic. The CC is particularly useful when there is heavy loading on best effort (DCF or EDCF) traffic, which makes it difficult for QSTAs to send TXOP requests through EDCF.
FIG. 1 is a schematic diagram of conventional IEEE 802.11e contention control. To begin a CC 12, the HC sends a contention control packet, which contains such information as the number of controlled contention transmission opportunities (Nccop) and the duration of each CCOP (Dccop). A contention control interval (CCI) duration 11a is obtained by Dccop multiplied by Nccop. After PIFS, each QSTA with a TXOP request randomly selects a particular CCOP to send its reservation request (RR) packet.